It was discovered only recently (Sutherland, 1974a) that human cells contain photoreactivating enzyme (PRE). This enzyme is known to repair lethal and mutagenic lesions from ultraviolet radiation (UV) in DNA by "splitting" cyclobuta-dipyrimidines ("pyrimidine dimers") with the help of light energy. The proposed work is based on earlier investigations concerning the mechanism of photoenzymatic repair (PR) and on my recent data showing that PRE from sonicates of human leukocytes permit PR of UV-irradiated Haemophilus transforming DNA in vitro in the presence of "photoreactivating light" (310 - 480 nm). It is intended: (1) to search for the presence of PRE in different cell types, using as a test the Haemophilus DNA assay system; determine the number of PRE molecules per cell; and characterize the biological, enzymatic and photochemical properties of the human PRE; (2) to demonstrate the occurrence of PR in UV-irradiated human leukocytes themselves (or possibly in other PRE-containing human cells) by the comparison of UV-irradiated cells after light or dark incubation with regard to their viability and the amount of photorepairable UV lesions remaining; (3) to assess the biological and medical significance of PRE in human cells by searching for possible correlations between PRE activity (including perhaps complete absence of it) and adverse effects on the human skin by sunlight, or technical UV sources for therapeutic, cosmetic, or sterilization use. (4) to study the expected overlap of PR with dark repair processes, which presumably play an impotant role in mitigating the potential damage by solar UV radiation to the exposed parts of the skin.